1. Field of the Invention
The present invention relates to electronics and, in particular, to clock-signal generating circuits.
2. Description of the Related Art
FIG. 1 shows a block diagram of a phase-locked loop (PLL) 100 of the prior art. PLL 100 includes a phase detector 102, a loop filter 104, a voltage-controlled oscillator (VCO) 106, and a feedback path having a frequency divider 108. A periodic reference signal 110 of frequency Fref is fed to phase detector 102 together with feedback signal 112 (the output of frequency divider 108). The output of phase detector 102 is a pulse that is related to the phase difference between reference signal 110 and feedback signal 112. The output of phase detector 102 is filtered through loop filter 104 and fed to VCO 106. Due to the feedback in the PLL, the frequency Fout of output signal 114 of VCO 106 is driven to equal the reference frequency Fref multiplied by the division factor of frequency divider 108, thereby resulting in a relatively high frequency of the output signal. Output signal 114 is typically a differential signal having two differential components.
One application of PLL 100 is to provide a reference clock signal, e.g., for a microprocessor or communication circuit. For this particular application, it is often desirable to design PLL 100 such that output signal 114 has a 50% duty cycle. In a 50% duty-cycle clock signal, the time interval between a positive transition edge and a negative transition edge is equal to the time interval between that negative transition edge and the next positive transition edge. In other words, all transition edges, regardless of the transition-edge direction, are equally spaced, which enables a half-rate (double-edge) clocking system. In contrast, in a clock signal having a duty cycle different from 50%, the time interval between a positive transition edge and a negative transition edge is different from the time interval between the negative transition edge and the next positive transition edge. Consequently, only one transition edge per cycle can be utilized as a valid reference point, which is known as a full-rate (single-edged) clocking system. Advantageously, a half-rate clocking system eases circuit design constraints compared to those of a full-rate clocking system because it effectively doubles the clock rate without having to double the corresponding VCO frequency.
PLL 100 is usually incorporated into a relatively large integrated circuit (chip) and manufactured using a suitable fabrication process, e.g., CMOS. However, technological limitations of the fabrication process often cause the duty cycle of PLL 100 to vary from chip to chip and deviate from the intended 50%. Due to relatively strict tolerances of certain half-rate clocking systems, a significant percentage of manufactured chips falls outside the acceptable duty-cycle range and has to be discarded.